Japanese Patent Application Laid-Open No. 2000-332351 discloses a two-dimensional photonic crystal surface-emitting laser having a two-dimensional photonic crystal placed near an active layer so as to achieve surface light emission by exploiting the resonance of the two-dimensional photonic crystal. The two-dimensional photonic crystal surface-emitting laser disclosed in this publication has a lower clad layer, an active layer, and an upper clad layer laid on a substrate. The lower clad layer incorporates a two-dimensional photonic crystal near the active layer.
The two-dimensional photonic crystal is produced by forming hollow holes in a semiconductor layer of, for example, n-type InP, and is formed as a triangular or square lattice having media having different refractive indices arrayed with a predetermined two-dimensional period. The hollow holes may be filled with SiN or the like. The active layer is formed as a multiple quantum well structure using, for example, an InGaAs/InGaAsP-based semiconductor material, and emits light when carriers are injected into it.
The lower clad layer is formed of, for example, an n-type InP semiconductor as described above, and the upper clad layer is formed of, for example, a p-type InP semiconductor. The active layer is sandwiched between the lower and upper clad layers to form a double hetero junction and thereby confine carriers so that the carriers that contribute to light emission concentrate in the active layer.
On the top surface of the upper clad layer and on the bottom surface of the substrate, there are formed electrodes of gold or the like. When a voltage is applied between the electrodes, the active layer emits light, and an evanescent component that leaks out of the active layer enters the two-dimensional photonic crystal. Light having a wavelength coincident with the lattice constant of the two-dimensional photonic crystal resonates with it, and is thereby amplified. As a result, the two-dimensional photonic crystal achieves surface light emission, emitting coherent light.
For example, in a two-dimensional photonic crystal formed as a square lattice as shown in FIG. 35, resonance occurs in the following manner. The two-dimensional photonic crystal 40 is formed as a square lattice having a second medium 12 in the form of hollow holes or the like formed with equal periods in two mutually perpendicular directions within a first medium 11. The square lattice has representative directions called the Γ-X and Γ-M directions, respectively. Let the interval between two patches of the second medium 12 that are mutually adjacent in the Γ-X direction (hereinafter, this interval will be referred to as the “lattice constant”) be “a,” then there exist a plurality of square lattice sections E1 having lattice points at patches of the second medium 12 and measuring “a” on each side (hereinafter, such a section will be referred to representatively as the “fundamental lattice”).
When light L having a wavelength “λ” coincident with the lattice constant “a” of the fundamental lattice E1 propagates in the Γ-X direction, the light L is diffracted at lattice points. Of the different components of the light, only those diffracted in the directions of 0°, ±90°, and 180° with respect to the direction of propagation of the light fulfill the Bragg condition. Lattice points exist also in the directions of propagation of the light that has been diffracted in the directions of 0°, ±90°, and 180°, and thus the diffracted light is diffracted again in the directions of 0°, ±90°, and 180° with respect to its direction of propagation.
When light L from one lattice point is refracted once or more than once, the diffracted light returns to the original lattice point. This causes resonance. On the other hand, the light diffracted in the direction perpendicular to the plane of the figure also fulfills the Bragg condition. As a result, light amplified through resonance is emitted through the upper clad layer, achieving surface light emission. This phenomenon occurs at every lattice point, permitting coherent laser emission all over the surface area.
FIG. 36 is a band diagram of the two-dimensional photonic crystal 40 structured as shown in FIG. 35. Along the vertical axis is taken the normalized frequency, i.e., the frequency of light normalized by being multiplied by “a/c,” where “c” represents the speed of light (in m/sec) and “a” represents the lattice constant (in m). Along the horizontal axis is taken the wave-number vector of light.
In this figure, the plotted lines indicate the dispersion relation of light. The figure shows that there are a few places in it where the gradient is zero. This means that there are a few points where the group velocity of light is zero and thus resonance occurs. In particular, at the point Γ, as described above, not only light diffracted in different directions within the plane but also light diffracted in the direction perpendicular to the plane fulfils the Bragg condition, and accordingly it is possible to extract, in the direction perpendicular to the plane, coherent light produced through resonance in different directions within the plane.
Incidentally, the point Γ is defined in the following manner. Let the unit vectors in a rectangular coordinate system be “x” and “y,” then the primitive translational vectors “a1” and “a2” with respect to a square lattice with a lattice constant “a” are given bya1=axa2=ay
For the translational vectors “a1” and “a2,” the primitive reciprocal lattice vectors “b1” and “b2” are given byb1=(2π/a)y b2=(2π/a)x 
On the basis of the primitive reciprocal lattice vectors “b1” and “b2,” the point at which the wave-number vector “k” of light has the value given by formula (1) below is called the Γ point.k=nb1+mb2  (1)where “n” and “m” are arbitrary integers.
Accordingly, at the Γ point, where the wave-number vector of light fulfills formula (1), the aforementioned phenomenon occurs in any band. In a two-dimensional photonic crystal surface-emitting laser, as indicated by the part S in the figure, it is typical to use the second order band, which corresponds to the case in which the lattice constant “a” is equal to the wavelength “λ.”
FIG. 37 shows the details of the part S. The two-dimensional photonic crystal has four frequencies, namely A, B, C, and D in order of increasing frequency, at which the group velocity is zero; that is, it has four resonant frequencies. Hereinafter, the resonant states at the resonant frequencies A, B, C, and D will be referred to as the modes A, B, C, and D, respectively.
FIGS. 38 and 39 show the electric field distributions observed when the two-dimensional photonic crystal is in the mode-A and mode-B resonant states, respectively. These images are the near field pattern images at the time of laser oscillation. Arrows indicate the directions and magnitudes of electric fields. As shown in these figures, in the modes A and B, the directions of electric fields are not uniform. That is, the polarization direction is not uniform. As a result, as shown in FIGS. 40 and 41, which show the electric field distributions in the far field pattern in the modes A and B, the polarization direction in the mode A is such as to run around an electrode 7, and the polarization direction in the mode B is such as to radiate to and from the electrode 7.
On the other hand, the modes C and D are degenerated so that those resonant states occur at the same frequency. Thus, at the point Γ, how polarization occurs is determined by the linear sum of the electric field distributions ascribable to the modes C and D. Thus, the polarization direction is not uniquely determined but remains unstable.
As described above, with a conventional two-dimensional photonic crystal surface-emitting laser, no matter in which of the four resonant modes (practically three, since the modes C and D are degenerated) of the two-dimensional photonic crystal it is formed to resonate, the polarization direction of the light emitted from it is not uniform. This makes the conventional two-dimensional photonic crystal surface-emitting laser unusable in devices that use uniformly polarized light.